The present invention generally relates to continuous type heat treating furnaces such as a continuous type gas carburizing furnace, a continuous type non-oxidizing hardening furnace, a continuous type annealing furnace, etc. and more particularly, to a continuous type gaseous atmosphere heat treating furnace employing a protective atmosphere.
In heat treatment such as carburizing, carbonitriding, non-oxidizing hardening, bright annealing, etc., it has been conventionally so arranged that a protective atmosphere suitable for heat treatment of ferrous metal works, for example, a carburizing gas, an endothermic gas, an exothermic gas, a mixture of the endothermic gas and the exothermic gas, etc. is drawn into the furnace such that the heat treatment is performed under the protective atmosphere. Continuous type atmosphere heat treating furnaces to be used for such heat treatment include a charge vestibule or a discharge vestibule designed for protecting atmosphere in the furnaces. The charge vestibule or the discharge vestibule, which is of a steel structure, is provided with a proper purge means and is substantially maintained at ambient temperatures. Accordingly, the known furnaces have such a drawback that in the case where the charge or discharge vestibule is subjected to gas purging at the time of transfer of the works from the charge vestibule to a heat treating chamber or transfer of the works from the heat treating chamber to the discharge vestibule, a purge gas in an amount four to six times a capacity of the charge or discharge vestibule is required to be used, thereby making the gas purging uneconomical. Furthermore, the known furnaces have such a disadvantage that since the works are heated from ambient temperatures in the heat treating chamber, the heat treating chamber itself is required to be made large in size, thus resulting in poor thermal efficiency of the heat treating chamber.
Meanwhile, in the case of gas carburizing, non-oxidizing hardening, etc. in which a combustible protective atmosphere is introduced into the heat treating chamber during the heat treatment, it has been so arranged that the atmosphere in the heat treating chamber is supplied into the charge or discharge vestibule so as to subject the charge or discharge vestibule to gas purging. In this case, the purge gas (combustible gas) is exhausted from the furnaces and burned off in the environment outside the furnace and thus, is not effectively utilized. Moreover, in gas carburizing, non-oxidizing hardening, etc., the works are washed by using trichloroethylene (trichlene) or are cleaned through heating thereof prior to loading of the works into the furnaces in order to remove from the works impurities such as oil, etc. adhering thereto. A cleaning apparatus for cleaning the works through heating thereof, i.e., for removing impurities such as oil, etc. from the works through heating thereof is proposed in Japanese Patent Publication No. 2588/1983. This prior art cleaning apparatus is separately provided forwardly of a continuous furnace so as to remove impurities such as cutting oil, etc. From the works through heating thereof. Subsequently, after the charge vestibule of the continuous furnace has been subjected to gas purging for about 20 to 30 min., the works are loaded into the heat treating chamber (gas carburizing chamber). Consequently, this prior art continuous furnace has such inconveniences that amount of the consumed purge gas increases, that heat possessed by the works at the time of cleaning of the works through heating thereof is not effectively utilized and that the production cost rises.